This invention relates to a method and apparatus for the generation and control of spin currents, comprising spin polarized charge carriers, in photoconductors using one or more coherent light beams. More particularly the present invention provides a method of using the polarization properties of multiple coherent light beams, and phase differences between multiple coherent light beams, to control the magnitude and direction of spin currents in a photoconductor.
The control of electronic spin in semiconductors is important for the study of spin dynamics in many-body systems and crucial for the development of new data storage and processing methods based on the spin degree of freedom of charged particles. This will be essential as a first step towards a solid state implementation of a quantum computer; see, e.g. D. D. Awschalom and J. M. Kikkawa, Phys. Today 52, No. 6, 33 (1999).
There has been considerable work on achieving spin-polarized currents in semiconductors using transport in the presence of magnetic impurities, see M. Oestreich et al. Appl. Phys. Lett. 74, 1251 (1999), R. Fiederling et al., Nature (London) 402, 787, (1999) and Y. Ohno et al., Nature (London) 402, 790, (1999), or using injection of carriers from a ferromagnetic contact, see P. R. Hammar et al., Phys. Rev. Lett. 83, 203 (1999), and S. Gardelis et al., Phys. Rev. B 60, 7764 (1999). In these cases a voltage applied across the semiconductor drives the spin current.
It is known that spin-polarized carriers can be optically injected into a semiconductor using circularly polarized light, see U.S. Pat. No. 3,968,376, and M. I. Dyakonov and V. I. Perel, in Optical Orientation, edited by F. Meier and B. P. Zakharchenya, Modern Problems in Condensed Matter Sciences, Vol. 8 (North-Holland, Amsterdam, 1984), Chapter 2. A spin current may be generated from these spin-polarized carriers by applying a voltage across the semiconductor, see D. Hagele et al., Appl. Phys. Lett. 73, 1580 (1980), and J. M. Kikkawa and D. D. Awschalom, Nature (London) 397, 139 (1999).
All of the above methods use a voltage difference to move the carriers (electrons and holes), and hence there is always an electrical current as well as a spin current. As well, the spin currents can only be modulated as fast as the voltage difference can be modulated.
U.S. Pat. No. 5,790,296 discloses a method for generating and controlling an electrical current in a semiconductor using the interference between multiple laser beams. This patent is restricted to the ways in which multiple light beams can be used to generate and control electrical currents, and does not discuss how to generate and control spin-polarized currents.
A single, elliptically polarized optical beam can be used to generate and control an electrical current in a semiconductor, only when the material has low enough symmetry (see, N. Laman, A. I. Shkrebtil, J. E. Sipe, and H. M. van Driel, Appl. Phys. Lett. 75, 2581 (1999)). In that case the current is controlled by the degree of elliptical polarization of the beam.
The production of pure spin currents have been discussed in the case of intersubband transitions in S. D. Ganichev, E. L. Ivchenko, S. N. Danilov, J. Eroms, W. Wegscheider, D. Weiss, and W. Prettl, Phys. Rev. Lett. 86, 4358 (2001. However, a drawback to this technique is that it requires a specially structured semiconductor material (quantum well) and can only be used at cryogenic temperatures with a far infrared laser.
It would therefore be very advantageous to provide a method of generating polarized spin currents in photoconductors that can be modulated on ultrafast timescales without the need for a bias voltage to be applied. It would also be advantageous to be able to use be able to generate spin currents using interband transitions (connecting valence and conduction bands) so that visible or near visible lasers could be used and so the use of far infrared lasers is not required, which could be accomplished at useful at room temperature and does not require artifical structuring of the semiconductor.
The present Invention provides a method of generating in photoconductors polarized spin currents that can be modulated on ultrafast timescales without the need for a bias voltage to be applied to the photonconductor.
In one aspect of the invention there is provided a method of generating spin currents in a photoconductor material, the method comprising the steps of:
producing a first coherent light beam having a first frequency xcfx891 and a second coherent light beam having a frequency twice the first frequency 2xcfx891 polarizing said first and second coherent light beams to have a preselected polarization with respect to each other, and simultaneously irradiating a selected region of the photoconductor material with said first coherent light beam and said second coherent light beam to excite charge carriers from a first energy band across a bandgap to another energy band thereby generating a spin current in said photoconductor having a direction and magnitude determined by the relative polarization of said first and second coherent light beams.
The present invention is not restricted to a requirement for two coherent light beams. Thus, in anther aspect of the invention there is provided a method of generating spin currents in a photoconductor material material having a bandgap energy separating two energy bands, the method comprising the steps of:
producing at least three coherent light beams of frequencies xcfx891, xcfx892, and xcfx893, such that xcfx891=xcfx892+xcfx893, polarizing each of said at least three coherent light beams to have a preselected polarization with respect to the other coherent light beams, at least xcfx891 having sufficient energy to excite charge carriers across the bandgap, and simultaneously irradiating a selected region of the photoconductor material with said at least three coherent light beams to generate a spin current in said photoconductor having a direction and magnitude determined by the relative polarization of said coherent light beams.
In the above aspects of the invention the method may include adjusting a phase relationship between the coherent light beams to change the direction of the spin current generated in the photoconductor.
In another aspect of the invention, the present invention provides a method of using a single optical beam with a degree of elliptical polarization incident on a semiconductor of low symmetry to achieve a spin current that can be generated and controlled by the degree of elliptical polarization. More particularly, in this aspect of the present invention there is provided a method of of generating spin currents in a photoconductor material which has a bandgap energy and non-centrosymmetric crystal symmetry, comprising the steps of:
illuminating a photoconductor material having a non-centrosymmetric crystal symmetry with a coherent light beam having a pre-selected degree of elliptical polarization and an effective photon beam energy and effective intensity to optically excite charge carriers from one energy band across said bandgap into another energy band, thereby generating a spin current in said photoconductor having a direction and magnitude determined by the degree of elliptical polarization of said coherent light beam.